1. Field of the Invention
The present invention relates to a reader using an image sensor such as a CCD (charge coupled device) sensor.
2. Description of the Related Art
Recently, a reader using a CCD sensor has been widely used as a means for inputting documents and graphic data to a computer, or an inputting means of each of a digital copying machine and a facsimile telegraph.
FIG. 9 is a block diagram of a CCD sensor used in such a reader. In this reader, signals of odd series analog shift registers and even series analog shift registers are finally synthesized in a line and outputted. Each of S.sub.1, S.sub.2, - - - , S.sub.N designates a light receiving portion. Each of OSR.sub.1, OSR.sub.2, - - - , OSR.sub.N/2 designates an odd side shift register for shift-out of an analog output of each of odd side light receiving portions arranged in an odd series. Each of ESR.sub.1, ESR.sub.2, - - - , ESR.sub.N/2 designates an even side shift register for shift-out of an analog output of each of even side light receiving portions arranged in an even series. BUFF designates a buffer amplifier. SH designates a start pulse for starting a shifting operation of each of the shift registers. Each of .phi..sub.1 and .phi..sub.2 designates a transfer pulse. .phi..sub.R designates a reset pulse. CCD.sub.OUT designates an output of a charge coupled device (CCD).
FIG. 5 shows a CCD sensor having two output channels. This CCD sensor can perform a scanning operation at a speed at least two times in comparison with the charge coupled device (CCD) shown in FIG. 9. Each of S.sub.1, S.sub.2, - - - , S.sub.N designates a light receiving portion. Each of OSR.sub.1, OSR.sub.2, - - - , OSR.sub.N/2 designates an odd side shift register for shift-out of an analog output of each of odd side light receiving portions arranged in an odd series. Each of ESR.sub.1, ESR.sub.2, - - - , ESR.sub.N/2 designates an even side shift register for shift-out of an analog output of each of even side light receiving portions arranged in an even series. OBUFF designates an odd side buffer amplifier. EBUFF designates an even side buffer amplifier. SH designates a start pulse for starting a shifting operation of each of the shift registers. Each of .phi..sub.1 and .phi..sub.2 designates a transfer pulse. Each of .phi..sub.RE and .phi..sub.RO designates a reset pulse. OCCD.sub.OUT designates an output of a charge coupled device (CCD) in each of the odd side light receiving portions. ECCD.sub.OUT designates an output of a charge coupled device (CCD) in each of the even side light receiving portions.
There is an image reader using such a CCD sensor having two output channels. In this image reader, each of an odd side output and an even side output is converted to a digital signal by an independent AID converter to increase a scanning speed.
FIG. 3 is an explanatory view for explaining the relation in position between a linear fluorescent lamp 33 and a CCD sensor 37 in a reader. In FIG. 3, an original 31 is arranged on a glass table 32. The original 31 is separated from the glass table 32 in this figure. However, in reality, the original 31 comes in close contact with the glass table 32. Light is emitted from the fluorescent lamp 33 arranged in a state in which a longitudinal direction of the fluorescent lamp 33 is perpendicular to a paper face. This light is transmitted through the glass table 32 and is irradiated onto the original 31. Light reflected on the original 31 is again transmitted through the glass table and is reflected on a mirror 35. Light reflected on the mirror 35 is converged by a lens 36 and is irradiated on a light receiving face of the CCD sensor 37. An optical unit 34 includes the mirror 35, the lens 38 and the CCD sensor 37. Reference numerals 38 and 39 respectively designate a pulse motor and a cabinet.
FIG. 4 is a block diagram of a general reader. In FIG. 4, reference numeral 40 designates the CCD sensor shown in FIG. 9. The CCD sensor 40 is connected to an input portion of a buffer 41 through a capacitor C. A power of 5 volts is connected to this input portion of the buffer 41 through a transistor ASW. An output portion of the buffer 41 is connected to an input portion of an AD converter 42. Terminal portions of outputs D.sub.O to D.sub.7 of the AD converter 42 are connected to a control circuit 43. A FLON output portion of the control circuit 43 is connected to an input portion of a lighting device 48. An output portion of the lighting device 48 is connected to an input portion of a fluorescent lamp 33. If FLON shows value "1", the fluorescent lamp 33 is turned on.
Terminal portions of outputs T.sub.STEP and FOWARD of the control circuit 43 are connected to an input portion of a pulse motor driving circuit 46. An output portion of the pulse motor driving circuit 46 is connected to an input portion of a pulse motor 38. When the pulse motor driving circuit 48 receives one T.sub.STEP pulse at the time of FOWARD="1", the pulse motor driving circuit 46 rotates the pulse motor 38 such that the optical unit 34 and the fluorescent lamp 33 shown in FIG. 3 are advanced by one step. Similarly, when the pulse motor driving circuit 46 receives the T.sub.STEP pulse at the time of FOWARD="0", the pulse motor driving circuit 48 rotates the pulse motor 38 such that the optical unit 34 and the fluorescent lamp 33 are retreated.
The control circuit 43 has an oscillator 44 and a clock generating circuit 45 connected to the oscillator 44. The oscillator 44 oscillates a basic clock signal t.sub.0. The clock generating circuit 45 divides a frequency of the basic clock signal t.sub.0 and generates various kinds of clock signals SH, .phi..sub.1, .phi..sub.2, .phi..sub.R and T.sub.CLAMP. The clock signals SH, .phi..sub.1, .phi..sub.2 and .phi..sub.R are transmitted to the CCD sensor 40. The clock signal T.sub.CLAMP is transmitted to a transistor ASW as an analog switch.
FIG. 2 shows timings of clock signals SH, .phi..sub.1, .phi..sub.2 and .phi..sub.R as driving signals transmitted to the CCD sensor 40, a CCD output signal CCD.sub.OUT , a signal T.sub.CLAMP for clamping the CCD output signal, and an input signal ADIN transmitted to the AD converter 42.
Operations of the transistor ASW and the capacitor C shown in FIG. 4 will next be described with reference to FIG. 10. No output CCD.sub.OUT of the CCD sensor 40 is normally determined with respect to a direct current (DC) voltage. As shown in FIG. 2 for example, the output CCD.sub.OUT is transmitted in a minus voltage direction with 4 volts as a reference. This output CCD.sub.OUT is dispersed from 4 V to a voltage value such as 3.5 V or 4.5 V in accordance with CCD sensors. However, the AD converter converts an analog potential equal to or lower than 5 V to a digital potential. Therefore, it is necessary to shift a DC voltage level such that a voltage equal to or lower than 5 V is outputted to the AD converter by the capacitor C and the transistor ASW. Accordingly, 1 volt is applied to the capacitor C at any time.
There is a load in a long transmission path of the output CCD.sub.OUT. Accordingly, an amplifier is normally used to amplify the output CCD.sub.OUT. Therefore, noises of this amplifier are included in this output CCD.sub.OUT at random. For example, the DC voltage level of the output CCD.sub.OUT is normally set to 4 V. However, when this DC voltage of the output CCD.sub.OUT is changed to 3.8 V by the above random noises as shown in FIG. 11, 1.2 V must be rapidly applied to the above capacitor C. Namely, 1.2 V greater than the normal 1 V by 0.2 V must be rapidly applied to the capacitor C. Accordingly, when a clamping time t is set to 300 nsec and a turning-on resistance R of the analog switch ASW is set to 50 .OMEGA., a capacity of the capacitor C satisfies the following condition. EQU C.times.R&lt;&lt;t EQU C.ltoreq.300 nsec/50.OMEGA.=6000p F
In a general scanner, the capacity of the capacitor C is selectively set to be equal to or smaller than 1000 pF. As is well known, an image having reduced random noises can be obtained in a reader by reliably clamping a DC voltage level of each of outputs of the CCD sensor. However, as shown in FIG. 2, it is necessary to further set a stabilizing period of the output CCD.sub.OUT before and after the clamp signal T.sub.CLAMP so that a basic clock signal is slowly transmitted and a scanning speed is finally reduced.
As mentioned above, an image having reduced random noises can be obtained in a reader by reliably clamping the DC voltage level of each of outputs of the CCD sensor. However, it is necessary to further set a stabilizing period of the output CCD.sub.OUT before and after the clamp signal T.sub.CLAMP so that an operating speed of the reader is reduced.
When two AD converters are used to increase the operating speed of the reader, AD-converting characteristics are different from each other in accordance with the AD converters so that periodic image stripes are caused on odd and even sides of the AD converters.